Conventional doorbells, doorchimes or other audible annunciators within a home or other building are typically hardwired to one or more pushbuttons or switches at an entrance door or doors and are activated by a person desiring entrance to the building, generating an audible sound, such as a chime tone, within the home or other building. Since the distance at which the sound of such a primary annunciator or device can be heard is limited, several such annunciators, in addition to the primary annunciator, may be required to alert an occupant of the building, e.g., in a basement, attic, garage or other remote area of the home or other building, as to the operation of the primary device. Since physically interconnecting or hardwiring remote devices to the primary device or to the pushbutton or switches is a complicated and expensive procedure, a different and more cost-effective approach is required.
Ways to activate remote annunciator devices without expensive wiring include transmitting an over-the-air, radio activating signal to a distant device. See, for example, U.S. Pat. No. 4,523,193, which describes a device for remotely activating a distant sound producer by such signal. These devices, however, typically operate by connecting the transmitter to the primary device circuit, thereby sharing a common power source. Thus, upon pressing the activating button and closing an electrical switch therein, the current flowing therethrough that energizes the primary device also energizes the transmitter. Connecting the transmitter of the activating signal so that the transmitter is energized by the power source for the primary device has disadvantages.
First of all, if there are two pushbuttons, at least three electrical connections must be made to the electrical circuit of the primary device which may be acceptable, in many cases, but it is desirable to provide an activating signal transmitter which can be used either with such connections or with fewer such connections. Furthermore, if the transmitter is energized by the same source which energizes the primary device, electrical problems, such as overload or voltage drop, may result.
Secondly, while older forms of annunciators which have solenoids which activate a mechanical sound generating device, such as a tone bar or a bell, will operate when the activating, or pushbutton switch is momentarily closed, there is a newer form of annunciator which includes electronic circuits which simulate the sounds of the older forms, e.g., a chime, and which are physically smaller and simpler. However, such newer form of annunciator requires a continuous supply of electrical energy, that is, even when the pushbutton switch is not closed, in order properly to function. If, in this case, the transmitter of the activating signal will transmit when the annunciator is energized, the transmitter will continuously transmit which, for obvious reasons, is unacceptable.
Thirdly, there is a limited band of radio frequencies in which an activating signal transmitter is permitted, by government regulation, to transmit. Thus, if there are at least two transmitters and two remote receivers in close proximity, within the transmitted power ranges of the transmitters, and the transmitters have substantially the same transmitting frequency, false activation of the remote annunciators can occur. For example, one transmitter can be in one building or apartment, another transmitter can be in another nearby building or apartment and all receivers can be near enough to both transmitters to activate both of their alerting devices, audible or visual, when either transmitter is activated. Since inexpensive receivers which are not highly selective are used, mere shifting of the transmission frequency of one of the transmitters within the limited band may not be sufficient to avoid false signalling.
There also are annunciators which produce audible sounds but which are entirely mechanical, i.e., they do not require an electrical power source or electrical energization, and it is desirable to be able to activate remote annunciators whenever such a non-electric, primary annunciator is activated.